In response to the ever increasing demand for improved fish growth rates and increased profitability, aquaculture farmers have turned to the use of high-protein fish feeds in large quantities. The use of high-protein feeds, however, leads to the generation of significantly increased amounts of nitrogenous waste (e.g., ammonia) due to the enhanced production of fish excretions (e.g., urea, uric acid, amino acids), organic debris from dead and dying organisms, and uneaten feed. Fish expel the increased nitrogenous waste products through gill diffusion, gill cation exchange, urine, and feces, which must therefore be removed from the aquaculture system. The decomposition of these nitrogenous compounds is particularly important in intensive recirculating aquaculture systems because of toxic effects of ammonia, nitrite, and, to a lesser extent, nitrate.
The process of ammonia removal by a biological filter is called nitrification, which consists of the successive oxidation of ammonia to nitrite (NO2−) and finally to nitrate (NO3−). This process is usually actuated by autotrophic bacteria via an aerobic biofilter process. There are many types of biofilters commonly used in aquaculture systems, such as submerged biofilters, trickling biofilters, rotating biological contactors, floating bead biofilters, dynamic bead biofilters, and fluidized-bed biofilters (see, e.g., U.S. Pat. Nos. 4,370,234; 4,620,924; 4,880,549; 5,445,740; 5,593,574; 5,770,080; 6,110,389; 6,617,155; 7,008,539; and 7,082,893; each incorporated herein in its entirety by this reference). However, each such filter is limited to specific applications, such as locations having fixed-position wastewater treatment facilities or having proximity to high-intensity industrial power and pressure sources, because of one or more of the following deficiencies: ineffective removal of ammonia; overproduction of nitrite; large spatial footprints; use of large amounts of power and/or pressure; high cost of operation and/or maintenance; lack of portability; capturing and/or trapping of solids (e.g., particulate matter or debris); enhancement of off-flavors in aquacultured fish; requirement for backwashing; ineffective reduction of supersaturated gas levels in the water; and lack of immediate functionality when transferred across different sources of water.
A biofilter that meets all or even most of these requirements does not currently exist. For example, biofilters using polyethylene moving bed media are commonly used in recirculating aquaculture systems, yet they are susceptible to forming sediment in undisturbed areas within the filter container. This contributes to unwanted off-flavor in fish and other organisms reared in recirculating aquaculture systems. Another disadvantage of existing moving bed biofilters is that they are not portable such that their use is limited to a single body of water. Another disadvantage of existing moving bed biofilters is that they are only effective after aerobic bacterial populations have become established, which is a time-consuming process that can require up to several weeks of bacterial growth.
Accordingly, there is a need in the art for enhanced biofilter systems and methods for removing nitrogenous wastes from bodies of water.